Crystal Sets -
an enduring miracle -- UPDATED
including details of a set you can build

This page gives an insight into the wondrous
world of crystal sets. Here you will find theoretical details of how a crystal
set works and the types of components used. You will also find details of how to
build a working set for yourself. You may have nostalgic recollections of a
crystal set that you owned or built in your youth or you may simply have
heard of such radios and wondered what they are all about; either way you will
find something of interest here. The more advanced reader may be interested in
the Popular Wireless
'Crystal Experimenter's Handbook' section below.

What is a Crystal Set?

A crystal set is the simplest form of AM
(amplitude modulation) receiver yet devised. It costs little to build, has great
potential for experimentation and usually requires no source of power for its
operation other than the radio signal itself. Does this sound too good to be true? Not at all. YOU can build one easily and
cheaply and I'm sure the results will impress. It will certainly give immense
satisfaction.

History

Crystal sets date back to the earliest days of
wireless (pre-W.W. I) and an enormous variety of circuit designs and cabinet
styles have been produced over the years. Their popularity has been variable as developments in
other more elaborate forms of reception have taken place. However peoples
fascination with this simplest form of reception is enduring and remains strong
to this day.

How do crystal sets work?

To answer this you
need to first understand a bit about the AM signal that is sent out from the transmitter.

An AM transmitter sends out its broadcast in
the form of an electromagnetic
wave that radiates from its aerial (antenna). With no sound content in the programme, the wave
is a constant amplitude 'carrier wave', who's frequency is as shown on the dial
of an AM radio (also sometimes shown as its wavelength). The strength, or
amplitude, of these waves is made to vary in response to the audio content of
the programme. The resulting wave is called an amplitude modulated wave. The
following diagram shows how such a wave would look if you could view its change
in amplitude with time:

The purpose of a simple AM receiver is to pick
up these AM waves and extract the audio signal so that it can be heard by the
listener. It does this by a process called 'detection'. The detection process
utilizes a device called a detector which effectively strips off either the upper half or lower half of the
AM wave. It
only remains to filter out the carrier wave to leave the audio signal and the
job is done! The following diagram shows how the detector and filter work
together to 'recover' the original audio signal:

The strength of the recovered audio signal is small but, given the
right conditions, it can be large enough to drive a pair of headphones; all
without any power source other than the signal itself. Amazing.

So what's required?

In its VERY simplest form, a crystal
set receiver comprises just four main elements:

1. A good aerial (antenna)
and earth

2. A detector

3. A filter capacitor (Cf)

4. An earpiece or headphones

These can combined to form the most basic
circuit, such as shown below:

Fig. 1 - Most basic crystal set

The aerial consists of a length of wire
suspended above the ground, whilst the earth may be a metal spike driven into the
ground. An AM broadcast will set up an alternating voltage between the aerial
and the earth. This voltage will cause an alternating current to flow around any
circuit connected between the aerial and earth. Thus an alternating current due
to the AM broadcast will try to flow through the detector. Since current can
only flow in one direction through the detector, the other half of the
alternating current is blocked. The remaining uni-directional current is
filtered by the combined effect of the headphone impedance and the capacitor and
only the audio signal current passes through the headphones.

One drawback with the very simple circuit
shown above is that it has little or no selectivity. That is, it picks up all AM
broadcast signals with similar efficiency. Some sort of tuning
is required so that we can select just the broadcast we want. The following
circuit has tuning components added:

Fig. 2 - Tuned crystal set

When a capacitor is connected in parallel with
an inductor and an alternating voltage applied across the combination,
alternating current will flow. The amount of current that flows depends upon the
frequency of the applied voltage. At a particular frequency, called the resonant
frequency, almost no current flows. For frequencies above or below the resonant
frequency, significant current will flow. In Fig. 2, C1 plus the aerial
capacitance forms the capacitor of our tuned circuit, whilst L1 forms the
inductor. At the resonant frequency of the tuned circuit, almost no current
flows to earth through L1 or C1, leaving virtually all of it free to flow
through the detector. Alternating currents at broadcast frequencies either above
or below the resonant frequency will tend to flow through C1 or L1 to earth.
Thus we have added selectivity to the receiver. Notice that C1 is adjustable, as
signified by the arrow. By varying C1 we can tune the receiver to select
specific broadcast frequencies.

The equation that relates (approximately) the
resonant frequency to the values of parallel capacitance and inductance is as
follows:

There are still a number of drawbacks with the
arrangement of Fig. 2:

1. The aerial forms a
significant part of the total tuning capacitance and so the tuning range
available by varying C1 will be small

2. A consequence of the
aerial capacitance is that the upper frequency that can be tuned is quite
limited.

3. The circuit is not
particularly efficient and tuning will be quite 'flat'.

We shall return to circuit topology later to
see what improvements can be made but first a bit more on 'the basics'. To skip
this and move straight to details of an improved design that you can easily
build click HERE. To return to
this page please use the 'back' button on your browser.

A bit more about the detector

The main requirement for a detector is that it
should act as a non-return 'valve' to the alternating currents of the AM wave.
If you could look at the alternating current in a circuit you would see it
flowing back and forth, first one way and then the other. A detector placed in
such a circuit allows the alternating current to flow easily in one direction but not in the other. Certain naturally
occurring minerals were found to have this property and these became some
of the
earliest forms of detector. One such mineral, the crystalline form of galena
(lead sulphide), was found to be particularly good at detection, and became very
popular. The following photograph shows a fine piece of galena in its original
tin. Also shown in the photograph is the 'cat's whisker' (the small coil of wire)
which was used to make contact with the crystal.

click on image to see
larger version

In use, the crystal was held securely in a
metal holder whilst the 'cat's whisker' was held in another small holder to which
was attached a handle. The photograph below shows such an arrangement, as used
on the GECoPHONE No. 1 crystal set:

click on image to see
larger version

The holder carrying the crystal forms one
electrical contact whilst the holder carrying the cat's whisker forms the other
contact. In use, it was necessary to carefully adjust the cat's whisker until
contact was made with one of the numerous contact sites covering the surface of
the crystal. Not all these contact sites are equally sensitive, and some
experimentation is required to find the 'sweet spot'. Unfortunately the
adjustment is not very stable and a slight knock or vibration could easily cause
contact to be lost. For this reason it was not used in receivers likely to
encounter rough use (e.g. military receivers).

Many other types of detector were found in the
early days of radio. Here is an extract from an excellent book 'Wireless The
Modern Magic Carpet' by Ralph Stranger*, Pub. Partridge, London 1928.
Chapter XI entitled 'Crystals' is an interesting insight into early work on
solid state physics. The material may also be of value to those experimenting
with different types of crystal detector. To download or view this pdf file
(603KB) click on the image of the book below. Note that to view this file you
will need Adobe Reader version 5.0 or later. To get Adobe Reader (entirely free)
please click on the link near the bottom of this page.

*real name: Ralph
Judson, a Russian-born engineer who joined the BBC in 1925, eventually becoming
Advertisement Manager. He wrote many useful and highly readable books. I have a
copy of 'Wireless the Modern Magic Carpet' signed by the author.

Some of these materials were used
by manufacturers to construct what were termed 'permanent' detectors, i.e.
detectors that required no adjustment. These must have been most attractive to
the home constructor. Today more modern forms of permanent detector suitable for
crystal sets are available, using a processed version of the mineral Germanium
as the means of detection.

If you are really keen on finding
out just what can be achieved using crystal and mineral detectors, have a look
at the Popular Wireless 'Crystal Experimenter's Handbook', a small but very
interesting and useful booklet which was originally given away free with the 3rd
October 1925 issue of Popular Wireless. To download or view the pdf file
(2.358MB) click HERE. To view
this file you will need Adobe Reader version 5.0 or later. To get Adobe Reader
(entirely free) please click on the link below:

For more on circuit topolgy and
details of a set that you can easily build, click
HERE.